This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Machine Ratings”.

1. Refer the diagram; a single-phase supply of 100V, 50Hz is fed to the step-up transformer having primary impedance of 1 ohms and secondary impedance 20 ohms. What is the power consumed by the load of 5 ohms?

a) 20 W

b) 40 W

c) 10 W

d) 100 W

View Answer

Explanation: The referred resistance to the secondary, Z2=1*(5

^{2}) = 25 ohms

Total resistance at secondary = 25+20 = 45 ohms

Total resistance seen by source = 45+5 = 50 ohms

Hence, current in secondary = 100/50 = 2 A

Power consumed by the load = I

^{2}*R = 4*5 = 20 W.

2. If the dimension of a single phase transformer is made sqrt(3) times less than the original structure, then what is new no load current rating of the transformer?

a) 1/sqrt(3)

b) sqrt(3)

c) 3

d) 1/3*sqrt(3)

View Answer

Explanation: The no load current of the transformer is directly proportional to the dimension of the transformer.

3. For a single phase, 50 Hz transformer, if the open circuit test is conducted at 40 Hz. What is the variation in the no-load power factor of the transformer?

a) Decreases

b) Increases

c) Remains constant

d) None of the mentioned

View Answer

Explanation: If the frequency reduces then flux density increases, so the flux and then the magnetizing current. When magnetizing current increases power factor angle increases so the power factor decreases.

4. The desired voltage regulation for a single phase, 50 Hz transformer is zero. Its pu ohmic drop is 4% and pu reactive drop is 4%. The required power factor angle for the operation in degrees is?

a) 45

b) 90

c) 30

d) 60

View Answer

Explanation: The power factor angle is, tan(phi)=pu resistive drop/pu reactive drop

= 4/4 = 1.

5. While conducting lab experiments on DC machine loading, the students are given a machine with its rating as 100 kW maximum rating. Then the _____________

a) machine can’t be overloaded after 100 kW

b) machine can be overloaded after 100 kW

c) machine can be overloaded by 5% for a small time duration

d) machine can be overloaded by 10%

View Answer

Explanation: As the maximum rating is specified and operation condition are also same, it cannot be overloaded beyond the maximum.

6. A 3-phase, five limbed shell type transformer, star connected, 200V, 50 Hz, 100 kVA will have ___________

a) no third harmonic current in its phase but has sinusoidal flux

b) third harmonic current in its phase but has flat-topped flux

c) no third harmonic current in its phase but has flat-topped flux

d) third harmonic current in its phase but has sinusoidal flux

View Answer

Explanation: Star connection in three phase has no closed path for the flow of the third harmonic current.

7. For a separately excited dc machine of 25 kW, 250 V and armature resistance of 0.25 ohms, is running at 3000 rpm with supply of 255 V. The electromagnetic power produced at the armature in kilo watts is?

a) 5

b) 10

c) 0.5

d) 8

View Answer

Explanation: The current in armature, Ia = 255-250/0.25 = 20 A

Power = Eb*Ia = 250*20 = 5000 W.

8. If two alternators A and B are operated in parallel having per unit synchronous reactance as 0.02 ohms and 0.05 ohms respectively.Then what the ratio of contributing loading of machine A to B?

a) 2.5

b) 0.4

c) 1

d) can not be operated in parallel

View Answer

Explanation: The load shared by the alternators is inversely related to their pu reactance.

9. A 200V, 10 hp, 4 poles, 60 Hz, Y-connected induction motor had full load slip of 5%. Then the rotor speed of the motor with respect to stator is?

a) 1710 rpm

b) 1800 rpm

c) 90 rpm

d) 3510 rpm

View Answer

Explanation: Synchronous speed, Ns = 120*f/P = 120*60/4 = 1800 rpm

Rotor speed = (1-slip)*Ns = 0.95*1800 = 1710 rpm.

10. Auto transformer rating is chosen so as to have best performance when the analogous two winding transformer has ___________

a) two voltage levels fairly close to each other

b) very high transformation ratio

c) any voltage levels

d) step down purpose

View Answer

Explanation: The auto transformer is best suited with the voltage levels of same rating for a two winding transformer.

11. In the lab, two students X and Y conducted the experiments to verify the name plate ratings of the transformer.But student X conducted the same at higher frequency. The kVA reading of student X will be ___________

a) higher than Y’s reading

b) lower than Y’s reading

c) same as Y’s reading

d) none of the mentioned

View Answer

Explanation: As rating is directly proportional to the frequency of operation.

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